In light of a passenger getting removed from a flight with another airline I am wondering what is a passenger to do when they have an "urgent" need to use the lavatory and the fasten seat belt sign is on? With the understanding the certain phases of a flight are critical and should not be ignored, what is SWA policy? I have had seen some flight crews allow quick trips and other impose the equivalent of martial law. Common sense would seem to apply but it is in what I would call short supply. I would never leave my seat when the sign is illuminated and there is noticeable instability on the flight.